Materials Science and Engineering A 445446 (2007) 16Effect of Nd and Y on the microstructure and mechanicalproperties of ZK60H.T. Zhoua, Z.D. Zhanga, C.M.alformAbstracttensileshograins. andthe withgrain combinationdynamicthe alloboth Nd andK1. IntroductionMg alloys are the lightest structural alloy, and hence theyaremotispecificstrengthrequirementsproperties.partsbacksaremechanicalbyhotmechanicalstrengthestill is low compared to aluminum alloy. From this point of view,many researchers devote their efforts to improve its mechanicalproperties. Recently, it is reported that extruded Mg alloys con-0921-5093/$doi:likely to be applicable to many structural parts in auto-ve and aero industries due to high specific strength, highstiffness and good damping capacity 1,2. However,of most current Mg alloys cannot meet the strengthof general structures because of some undesirableTherefore, applications of Mg alloys as structuralare still very limited. In order to overcome these draw-and widen the application fields of Mg alloys, researcherstrying any kinds of methods. It has been demonstrated thatproperties of Mg alloys are significantly improvedgrain refinement through adding rare earth metals (RE) andworking 3,4. As well known, ZK60 alloy has highestproperties among all the Mg alloys such as highat room temperature and elevated temperature 5.How-ver, its strength at room temperature and elevated temperatureCorresponding author. Tel.: +86 731 8830257； fax: +86 731 8830257.E-mail address: (H.T. Zhou).taining RE exhibit excellent mechanical properties 6,7.Forinstance, Ma et al. studied on extruded ZK60-RE alloys andsuggested that hot extrusion could improve tensile properties ofZK60-RE 8. Singh and Tsai 9 and Zhang et al. 10 studiedthe effect of Y on microstructure and mechanical properties ofZK60 alloy. They point out that Y enhances the yield strengthand elevated temperature strength by forming new phases of(WMg3ZnY2) and (IMg3Zn6Y) which have high hardiness,thermal stability, high corrosion resistance, low coefficient offriction, low interfacial energy, etc. 11,12. Subsequently, thesenew phases can effectively obstruct the slip of dislocation duringhot deformation. Although the mechanical properties of ZK60alloy could be improved by an addition of Y, the expected prop-erties is not reachable. Therefore, in this study, we initializethis article to study the effects of Nd addition and combinationaddition of Nd and Y on microstructure and tensile propertiesof ZK60 alloy. Furthermore, the relationship between the ten-sile properties and microstructure is investigated in hot extrudedalloys. see front matter 2006 Published by Elsevier B.V.10.1016/j.msea.2006.04.028aSchool of Materials Science and Engineering, CentrbSchool of Materials Science and Engineering, ShanghaiReceived 26 July 2005； received in revisedThe effect of neodium (Nd) and yttrium (Y) on the microstructure andw that an addition of neodium and yttrium both brings about precipitationAfter hot extrusion, the alloy added with Nd and the alloy with Ndpining effect of particles or precipitates. As a result, very finer grainssize of the alloy with Nd is relatively large. This suggested that therecrystallization, and leads to either the increase of both the meltingalloys or the increase of the yield strength and tensile strength of theZK60 and the alloy with Nd are higher than that of the alloy with2006 Published by Elsevier B.V.eywords: ZK60 alloy； Neodium； Yttrium； Extrusion； Tensile propertiesalloyLiua, Q.W. WangbSouth University, Changsha 410083, PR ChinaJiao Tong University, Shanghai 200030, PR China29 March 2006； accepted 12 April 2006properties of ZK60 alloy are investigated. Experimental resultsof a new Mg41Nd5and Mg3Zn6Y (I) phases and refine the as-castY are greatly refined through dynamic recrystallization by means ofsize of 48H9262m are obtained in the alloy with Nd and Y. However, theof Nd and Y addition has a great effect on grain refining duringtemperatures of the eutectic phases and the melting temperature ofy with Nd and Y at room temperature. In contrast, the elongation ofY.2 H.T. Zhou et al. / Materials Science and Engineering A 445446 (2007) 16Table 1Chemical compositionComposition (wt.%)Mg Zn Zr Y NdAlloy A Bulk 5.54 0.54AlloAllo2.Tmixthepurring,tointointo420atspecimensfrom10arephasedifHNO3.3.1.respecticomposedphase.marilyasMeanwhile,allosizeasbeandNdconstantallotripleindicates(inmuchinbyFig. 1. Microstructure in as-cast: (a) A alloy, (b) B alloy and (c) C alloy.phase and I phase (Mg3Zn6Y), further identifying C alloy hasI phase (Mg3Zn6Y, icosahedral quasicrystal structure) exceptMg41Nd5. The formation of cluster compounds can be ascribedto the increase of total amount of Nd and Y 14. However, Wphase (Mg3Zn3Y2) and Z phase (Mg12ZnY) cannot be found byy B Bulk 5.53 0.55 2.13y C Bulk 5.56 0.53 1.54 2.14Experimental proceduresThe chemical compositions of the studied alloys are listed inable 1. The alloys was prepared in furnace under protection of aed gas atmosphere of SF6 (1 vol.%) and CO2(BAL). Whenmolten alloy reaches 780C, it is purred for about 300 s. Afterthe molten alloy is hold for 15 min to allow inclusionssettle to the bottom of the crucible. Then, the metal is poureda medium furnace. At 680C, the molten metal is pouredingots with size of 90 mm. The ingots are solutionzed atC for 18 h. They are extruded into long rods of 20 mm390C, respectively, with an extrusion ratio of 20:1. Tensileof 5 mm diameter and 66 mm length are machined outthese extruded rods. The size of tensile testing specimens ismm wide and 66 mm long. The microstructure of specimensanalyzed by a light microscopy (OM, LEICA MEF4M), andanalysis is performed by means of a D/MAS-IIIA X-rayfractometer (XRD). All the specimens are etched with 4%3solution in alcohol.ResultsMicrostructure of as-cast ZK60 alloysFig. 1 shows microstructures of as-cast A, B and C alloys,vely. It can be seen from Fig. 1a that A (ZK60) alloy isof primary H9251 (Mg) matrix and eutectic H9252 (Mg2Zn3)The H9252 phase precipitates as discontinuous network pri-at grain boundaries. When there is Nd addition namedB alloy, more second phase precipitated, as shown in Fig. 1b.Nd and Y are added together to ZK60 alloy called Cy, it seems to be that much more compounds appear, and theof the compounds is smaller than that of A and B alloysshown in Fig. 1c. Consequently, different grain sizes canfound among A, B and C alloys in the sequence of 90, 6040H9262m, respectively. Therefore, it could be concluded thatand Y have an effect on refinement of ZK60 alloy. This iswith the result of Luo 13.Fig. 2 shows SEM microstructure images of B and Cys. It is found that there are some cluster compounds atgrain boundaries as seen A of Fig. 2a. EDAX analysisthat its chemical composition formula is Mg41Nd5nMg:nNd= 1.25:0.14/10.8:1.2). This is conformed by XRD seenFig. 3. When Nd and Y are added together into ZK60 alloy,more cluster compounds appear at triple grain boundarieswhich there are some paralleled laths. They are identifiedXRD for C alloy. It can be seen that there exists Mg41Nd5XRDadditionsurpassed(Mgand EDAX analysis in this experiment. Clearly, yttriumbrought about the formation of I phase in alloy C, andthe formation of W phase (Mg3Zn3Y2) and Z phase12ZnY).H.T. Zhou et al. / Materials Science and Engineering A 445446 (2007) 16 3(a) BandandissecondarythecertainfurtherfoundofsecondalloofasofandThistemperature(340furtherincreases3.2.wereoccurredsizeInphasethatatsmall,foundofthatrolehand,arepinningfinebeis3.3.eultimateB(Aalloproofandwheretosize.BincreaseB4.Fig. 2. SEM images:Fig. 4 shows the map distribution of B and C alloys for NdY. It is found that Nd and Y exist both in grain boundariesin matrix. However, in some area, the content of Nd or Yvery high. As seen from Fig. 4a and c, it suggests that thephases are likely to contain more Nd or yttrium thanmatrix. This may be used to explain the phenomenon thatamount of secondary phases exists in alloys B and C. Thisconforms the results are agreement with XRD results.Fig. 5 shows the DTA analysis results of alloys B and C. It isthat the first endothermic peak appeared at the temperatureabout 463.3C for alloy B and 485.7C for alloy C, while thepeak appeared at 617.5C for B alloy and 615C for Cy The first peaks can be thought as the melting temperaturesthe eutectic phases, and the second peaks can be thoughtthe melting temperature of the alloys (solution temperaturealloy). We can conclude that the combined addition of NdY to ZK60 alloy greatly increases the eutectic temperature.is agreement with yttrium can greatly increase the eutecticover the eutectic temperature of MgZn binary alloyC) 10,15. Results of the DTA analysis of this experimentsuggest that the eutectic temperature of MgZnZr alloywith increasing total content of Nd and Y.Microstructure evolution of the hot extruded alloysFig. 6 shows optical microstructures of alloys A, B and C thatextruded at 390C. It is found that all the three alloys havedynamic recrystallization (DRX). However, the grainand the amount distribution of second phase are different.hot extruded alloys A as shown in Fig. 6a, there is no secondon the matrix. The size of DRX grain is large compared toof alloys B and C. There seems a little growth of grains eventhis temperature. In the alloys B and C, DRX grain size is veryand DRX also completed. Full details on matrix can bewith characteristics of some second phases. The grain sizealloy C is the smallest among the three alloys. This suggeststhe combined addition of Nd and Y plays an importantduring process of dynamic recrystallization. On the otherDRX grains of alloy C with an average size of about 4H9262mcasttensileH9251andtionallobyreactiontionliquid/solidthevalloy and (b) C alloy.very fine and uniform. This may relate to the fact that theeffects of both broken secondary phase particles andprecipitates can suppress the growth of DRX grains. It canconcluded that grain refinement by dynamic recrystallizationvery effective even at this temperature in ZK60 alloy.Mechanical properties of extruded alloysFig. 7 shows the mechanical properties of the threextruded alloys at 390C. As shown in the figures, thetensile strength and yield strength of alloys A,and C increase, while the ductility of them decreasesalloy: 0.2= 270.2 MPa, b= 320.5 MPa, = 12%； By: 0.2= 316.2 MPa, b= 373.2 MPa, = 8%； C alloy:0.2= 376.2 MPa, b= 389.0 MPa, = 6%). Clearly, the 0.2%of stress strongly depends on the grain size in Mg alloy 16obey the role of HallPetch relationship y= 0+ Kd1/2,yis the yield stress, 0the lattice friction stress relatedmove individual dislocation, K a constant and d is the grainThus, this can explain why the tensile properties of alloysand C are higher than that of A (ZK60) alloy. In addition, theof ultimate tensile strength and yield strength of alloysand C may be related to the second phase strengthening.DiscussionAlloys A, B and C have different microstructure both in as-and in extruded state. Subsequently, it brings to differentproperties. Firstly, as in as-cast state, alloy A consists ofMg and H9252 (Mg2Zn3) phase. When Nd is added into A alloy,Nd with Y is together added into A forming C alloy, in addi-toH9252(Mg2Zn3) phase, the new phases appear as Mg41Nd5iny B, and Mg41Nd5+Mg3Zn6Y in alloy C. This is identifiedXRD and SEM. During solidification process the peritecticshould occur first. Owing to the no equilibrium distribu-the solute atoms of Zn and RE are pushed to the front of theinterface formed along the grain boundaries while ininterior of the grain only the Zr-rich zone is present. This iserified by Fig. 4. From Fig. 1, we conclude that the grain refin-4 H.T. Zhou et al. / Materials Science and Engineering A 445446 (2007) 16ingagreementeiningacrossrelatigraintheingFig. 3. XRD diffraction: (a) A alloy, (b) B alloy and (c) C alloy.effect of Nd and Y element exists in Mg alloy. This is goodwith experiment observations 17,18. Secondly, as inxtruded state, Mg41Nd5in alloy B, and Mg41Nd5+Mg3Zn6Yalloy C are destroyed and broken into small particles. Dur-hot extrusion, many fine particles homogeneously distributethe matrix. These thermally stable second phases with avely high meting point can pin grain boundaries and impedegrowth during hot deformation, especially I phase. Due tolow interfacial energy of I phase/matrix interface, the bond-at the I phase/interface is relatively strong 12 so that I phaseFig.anddeformation.ondlarareadiameterby4. Map distribution: (a) Nd alloy B, (b) Nd alloy C and (c) Y alloy C.precipitates were relatively difficult to be moved during hotThirdly, concentrated strain in the vicinity of sec-phases can introduce sites of high dislocation density andge orientation gradient (particle deformation zone). Such sitesideal for nucleation of recrystallized grains. It is known thatparticle deformation zone may extend to a distance of even onefrom the surface of the particles and may be disorientedtens of degrees from the adjacent matrix. In these deformationH.T. Zhou et al. / Materials Science and Engineering A 445446 (2007) 16 5zones,grainsmotedsecondgrovparticlesBtheonetheonDelta1wherement,thethan5.Mg6Zn0.5Zr2NdstudiedprecipitationrefineaddeddynamicticlesFig. 5. DTA trace of as-cast: (a) alloy B and (b) alloy C.second particles can stimulate nucleation of recrystallized19,20. Thus, nucleation of recrystallization can be pro-by Nd and Y addition in ZK60 alloy through formingphases. In addition, the second phases can hinder grainwth during recrstallization 20. As a result, alloy C exhibitsery finer grains. This is attributed to much more disperse finerthan alloys A and B. Therefore, the strength of alloysand C is much higher. This suggests that second phase, excepteffect of grain refining, has a strong strengthening effectthe strength of MgZnZr alloys, especially that the I phasexhibit obviously a strong strengthen effect 10. According towell known HallPetch relation, the yield strength dependsthe grain size as follows 16:0.2= Kd1/2(1)Delta10.2is the increase in yield stress due to grain refine-K a constant and d is the grain size. So, grain refinement byDRX process has an influence on alloys B and C are higherthat of ZK60 alloy.SummaryThe microstructure and mechanical properties of ZK60,and Mg6Zn0.5Zr2Nd1.5Y alloys arein this article. Some neodium and yttrium brings aboutof a new Mg41Nd5and Mg3Zn6Y (I) phases andthe as-cast grains with an addition of Nd and Y. The alloywith Nd and the alloy with Nd and Y are refined throughrecrystallization by means of the pining effect of par-or precipitates. This suggested that the combination ofFig.(c)Nddynamicbothmeltingstrength6. Optical microstructures extruded at 390C: (a) A alloy, (b) B alloy andC alloy.and Y addition has a great effect on grain refining duringrecrystallization, and leads to either the increase ofthe melting temperatures of the eutectic phases and thetemperature of the alloys or the increase of the yieldand tensile strength of the alloy with Nd and Y at room6 H.T. Zhou et al. / Materials Science and Engineering A 445446 (2007) 16Fig. 7. The tensile properties of the extruded alloys at 390C: A alloy, B alloyand C alloy.temperature. In c